Photovoltaic device with scratch-resistant coating

ABSTRACT

A method of making an anti-reflection coating using a sol-gel process, for use in a photovoltaic device or the like. The method may include the following steps in certain example embodiments: forming a polymeric component of silica by mixing silane(s) with one or more of a first solvent, a catalyst, and water; forming a silica sol gel by mixing the polymeric component with a colloidal silica, and optionally a second solvent; forming a metal oxide sol by mixing silane(s) with a metal oxide, a second catalyst, and a third solvent; forming a combined sol by mixing the metal oxide sol with the silica sol; casting the mixture by spin coating or the like to form a silica and metal oxide containing layer on a substrate; and curing and/or heat treating the layer. This layer may make up all or only part of an anti-reflection coating which may be used in a photovoltaic device or the like.

This application is a divisional of Application Ser. No. 11/716,034,filed Mar. 9, 2007, now U.S. Pat. No. 7,767,253 the entire disclosure ofwhich is hereby incorporated herein by reference in this application.

This invention relates to a method of making an antireflective (AR)coating supported by a glass substrate for use in a photovoltaic deviceor the like. The AR coating includes, in certain exemplary embodiments,metal oxide(s) and porous silica, which may be produced using a sol-gelprocess. The AR coating may, for example, be deposited on glass used asa superstrate for the production of photovoltaic devices, although italso may used in other applications.

BACKGROUND OF THE INVENTION

Glass is desirable for numerous properties and applications, includingoptical clarity and overall visual appearance. For some exampleapplications, certain optical properties (e.g., light transmission,reflection and/or absorption) are desired to be optimized. For example,in certain example instances, reduction of light reflection from thesurface of a glass substrate may be desirable for storefront windows,display cases, photovoltaic devices such as solar cells, picture frames,other types of windows, and so forth.

Photovoltaic devices such as solar cells (and modules therefor) areknown in the art. Glass is an integral part of most common commercialphotovoltaic modules, including both crystalline and thin film types. Asolar cell/module may include, for example, a photoelectric transferfilm made up of one or more layers located between a pair of substrates.One or more of the substrates may be of glass, and the photoelectrictransfer film (typically semiconductor) is for converting solar energyto electricity. Example solar cells are disclosed in U.S. Pat. Nos.4,510,344, 4,806,436, 6,506,622, 5,977,477, and JP 07-122764, thedisclosures of which are hereby incorporated herein by reference.

Substrate(s) in a solar cell/module are sometimes made of glass.Incoming radiation passes through the incident glass substrate of thesolar cell before reaching the active layer(s) (e.g., photoelectrictransfer film such as a semiconductor) of the solar cell. Radiation thatis reflected by the incident glass substrate does not make its way intothe active layer(s) of the solar cell, thereby resulting in a lessefficient solar cell. In other words, it would be desirable to decreasethe amount of radiation that is reflected by the incident substrate,thereby increasing the amount of radiation that makes its way to theactive layer(s) of the solar cell. In particular, the power output of asolar cell or photovoltaic (PV) module may be dependant upon the amountof light, or number of photons, within a specific range of the solarspectrum that pass through the incident glass substrate and reach thephotovoltaic semiconductor.

Because the power output of the module may depend upon the amount oflight within the solar spectrum that passes through the glass andreaches the PV semiconductor, certain attempts have been made in anattempt to boost overall solar transmission through glass used in PVmodules. One attempt is the use of iron-free or “clear” glass, which mayincrease the amount of solar light transmission when compared to regularfloat glass, through absorption minimization.

In certain example embodiments of this invention, an attempt to addressthe aforesaid problem(s) is made using an antireflective (AR) coating ona glass substrate (the AR coating may be provided on either side of theglass substrate in different embodiments of this invention). An ARcoating may increase transmission of light through the substrate so thatadditional light reaches the semiconductor absorber film of the PVdevice. Thus, the power of a PV module can be improved in certainexample embodiments of this invention.

It is known to use porous silica as AR coating on a glass substrate. Butan AR coatings made solely from porous silica may be ineffective withrespect to scratch resistance in certain instances. Such a low scratchresistance may be caused by the presence of pores, especially a largenumber of pores, in the AR coating. There may be a need to increase thescratch resistant property of AR coatings at any given transmission forhigh performance AR coatings that can be used as a PV superstrate.

Thus, it will be appreciated that there may exist a need for an improvedAR coating, for solar cells or other applications, to reduce reflectionoff glass and other substrates.

BRIEF SUMMARY OF EXAMPLE EMBODIMENTS OF THE INVENTION

Certain example embodiments of this invention relate, in part, to theformulation and manufacture of AR coatings produced using a sol-gelprocess, which are based on porous silica containing metal oxide(s), foruse in connection with glass intended to be used as a substrate in aphotovoltaic device or the like. These porous silica coatings may havehigh transmittance, thereby improving the efficiency and/or power of thephotovoltaic device in certain example embodiments. These coatings mayalso have improved scratch resistance.

In certain example embodiments of this invention, there is provided amethod of making an anti-reflection coating for use in a photovoltaicdevice or the like, the method comprising: forming a polymeric componentof silica by mixing at least a silane with one or more of a firstsolvent, a catalyst, and water; forming a sol gel by mixing thepolymeric component with a colloidal silica, and optionally a secondsolvent; forming a metal oxide sol by mixing a silane with a metaloxide, a catalyst, water, optionally a complex agent, and optionally athird solvent; mixing the silica sol with the metal oxide sol; castingthe mixture by spin coating to form a layer on a glass substrate; andcuring and/or heat treating the layer, the layer making up at least partof the anti-reflecting (AR) coating.

The metal oxide(s) used in making the porous silica based layer (e.g.,see layer 3 a in the figures) are advantageous in that they may permitthe resulting scratch resistance of the final layer 3 a to be greater orincreased. These metal oxide(s) may allow the materials of the poroussilica based layer to react in order to increase the hardness of theresulting layer, which is advantageous.

In certain exemplary embodiments of this invention, there is provided amethod of making an anti-reflection coating using a sol-gel processincluding: forming a polymeric component of silica by mixingglycycloxypropyltrimethoxysilane with a first solvent, a first catalyst,and water; forming a silica sol gel by mixing the polymeric componentwith a colloidal silica and a second solvent; forming a metal oxide solby mixing at least glycycloxypropyltrimethoxysilane with a metal oxide,a second catalyst, and a third solvent; forming a combined sol gel bymixing the silica sol with the metal oxide sol; casting the combined solgel by spin coating to form a coating on a substrate; and curing or heattreating the coating.

In certain exemplary embodiments, there is a method of making a coatingfor deposition on a substrate including: forming a combined sol gel bymixing a silane with a solvent, a catalyst, water, a colloidal silica,and a metal oxide; casting the combined sol gel by spin coating to forma layer on the substrate; and curing or heat treating the layer.

In certain exemplary embodiments, there is a photovoltaic deviceincluding a photovoltaic film, and at least a glass substrate on a lightincident side of the photovoltaic film; an anti-reflection coatingprovided on the glass substrate; wherein the anti-reflection coatingcomprises at least a layer provided directly on and contacting the glasssubstrate, wherein the layer comprises a metal oxide. Optionally, theglass substrate comprises a soda-lime-silica glass including thefollowing ingredients: SiO₂, 67-75% by weight; Na₂O, 10-20% by weight;CaO, 5-15% by weight; MgO, 0-7% by weight; Al₂O₃, 0-5% by weight; K₂O,0-5% by weight; Li₂O, 0-1.5% by weight; and BaO, 0-1%, by weight.Optionally, the layer provided directly on and contacting the glasssubstrate comprises porous silica and/or 3Al₂O₃:2SiO₂, Al₂O₃: SiO₂,MgAl₂O₄, ZrO₂, Al₂O₃, or CaAl₂Si₂O₈.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a coated article including anantireflective (AR) coating made in accordance with an exampleembodiment of this invention (this coated article of FIG. 1 may be usedin connection with a photovoltaic device or in any other suitableapplication in different embodiments of this invention).

FIG. 2 is a cross-sectional view of a photovoltaic device that may usethe AR coating of FIG. 1.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now more particularly to the accompanying drawings in whichlike reference numerals indicate like parts throughout the severalviews.

This invention relates to antireflective (AR) coatings provided forcoated articles that may be used in devices such as photovoltaicdevices, storefront windows, display cases, picture frames, other typesof windows, and the like. In certain example embodiments (e.g., inphotovoltaic devices), the AR coating may be provided on either thelight incident side or the other side of the substrate (e.g., glasssubstrate).

Photovoltaic devices such as solar cells convert solar radiation intousable electrical energy. The energy conversion occurs typically as theresult of the photovoltaic effect. Solar radiation (e.g., sunlight)impinging on a photovoltaic device and absorbed by an active region ofsemiconductor material (e.g., a semiconductor film including one or moresemiconductor layers such as a-Si layers, the semiconductor sometimesbeing called an absorbing layer or film) generates electron-hole pairsin the active region. The electrons and holes may be separated by anelectric field of a junction in the photovoltaic device. The separationof the electrons and holes by the junction results in the generation ofan electric current and voltage. In certain example embodiments, theelectrons flow toward the region of the semiconductor material havingn-type conductivity, and holes flow toward the region of thesemiconductor having p-type conductivity. Current can flow through anexternal circuit connecting the n-type region to the p-type region aslight continues to generate electron-hole pairs in the photovoltaicdevice.

In certain example embodiments, single junction amorphous silicon (a-Si)photovoltaic devices include three semiconductor layers. In particular,a p-layer, an n-layer and an i-layer which is intrinsic. The amorphoussilicon film (which may include one or more layers such as p, n and itype layers) may be of hydrogenated amorphous silicon in certaininstances, but may also be of or include hydrogenated amorphous siliconcarbon or hydrogenated amorphous silicon germanium, or the like, incertain example embodiments of this invention. For example and withoutlimitation, when a photon of light is absorbed in the i-layer it givesrise to a unit of electrical current (an electron-hole pair). The p andn-layers, which contain charged dopant ions, set up an electric fieldacross the i-layer which draws the electric charge out of the i-layerand sends it to an optional external circuit where it can provide powerfor electrical components. It is noted that while certain exampleembodiments of this invention are directed toward amorphous-siliconbased photovoltaic devices, this invention is not so limited and may beused in conjunction with other types of photovoltaic devices in certaininstances including but not limited to devices including other types ofsemiconductor material, single or tandem thin-film solar cells, CdSand/or CdTe photovoltaic devices, polysilicon and/or microcrystalline Siphotovoltaic devices, and the like.

In certain example embodiments of this invention, an improved AR coatingis provided on an incident glass substrate of a photovoltaic device suchas a solar cell or the like. This AR coating may function to reducereflection of light from the glass substrate, thereby allowing morelight within the solar spectrum to pass through the incident glasssubstrate and reach the photovoltaic semiconductor film so that thedevice can be more efficient. In other example embodiments of thisinvention, such an AR coating is used in applications other thanphotovoltaic devices, such as in storefront windows, display cases,picture frames, other types of windows, and the like. The glasssubstrate may be a glass superstrate or any other type of glasssubstrate in different instances.

FIG. 1 is a cross sectional view of a coated article according to anexample embodiment of this invention. The coated article of FIG. 1includes a glass substrate 1 and an AR coating 3. The AR coatingincludes a first layer 3 a and an optional overcoat layer 3 b.

In the FIG. 1 embodiment, the antireflective coating 3 includes firstlayer 3 a of or including porous silica, which is produced using thesol-gel process including metal oxide(s). These metal oxides may, forexample, be selected from metal oxides having a refractive index lowerthan about 2.0 (such as, for example: aluminum oxides like mullite,sillimanite, and alumina; magnesium-aluminum oxides like spinel;zirconium oxides like zirconia; calcium-aluminum oxides likeanorthrite). The first layer 3 a may be any suitable thickness incertain example embodiments of this invention.

Optionally, the AR coating 3 may also include an overcoat 3 b of orincluding material such as silicon oxide (e.g., SiO₂), or the like,which may be provided over the first layer 3 a in certain exampleembodiments of this invention as shown in FIG. 1. The overcoat layer 3 bmay be deposited over layer 3 a in any suitable manner. For example, aSi or SiAl target could be sputtered in an oxygen and argon atmosphereto sputter-deposit the silicon oxide inclusive layer 3 b. Alternatively,the silicon oxide inclusive layer 3 b could be deposited by flamepyrolysis, or any other suitable technique such as spraying, rollcoating, printing, via silica precursor sol-gel solution (then dryingand curing), coating with a silica dispersion of nano or colloidalparticles, vapor phase deposition, and so forth. It is noted that it ispossible to form other layer(s) over overcoat layer 3 b in certainexample instances. It is also possible to form other layer(s) betweenlayers 3 a and 3 b, and/or between glass substrate 1 and layer 3 a, indifferent example embodiments of this invention.

It is noted that layer 3 a and/or 3 b may be doped with other materialssuch as titanium, aluminum, nitrogen or the like.

In certain example embodiments of this invention, high transmissionlow-iron glass may be used for glass substrate 1 in order to furtherincrease the transmission of radiation (e.g., photons) to the activelayer(s) of the solar cell or the like. For example and withoutlimitation, the glass substrate 1 may be of any of the glasses describedin any of U.S. patent application Ser. Nos. 11/049,292 and/or11/122,218, the disclosures of which are hereby incorporated herein byreference. Furthermore, additional suitable glasses include, for example(i.e., and without limitation): standard clear glass; and/or low-ironglass, such as Guardian's ExtraClear, UltraWhite, or Solar. No matterthe composition of the glass substrate, certain embodiments ofanti-reflective coatings produced in accordance with the presentinvention may increase transmission of light to the active semiconductorfilm 5 (one or more layers) of the photovoltaic device and/or have adesirable or improved resistivity to scratching.

Certain glasses for glass substrate 1 (which or may not be patterned indifferent instances) according to example embodiments of this inventionutilize soda-lime-silica flat glass as their base composition/glass. Inaddition to base composition/glass, a colorant portion may be providedin order to achieve a glass that is fairly clear in color and/or has ahigh visible transmission. An exemplary soda-lime-silica base glassaccording to certain embodiments of this invention, on a weightpercentage basis, includes the following basic ingredients: SiO₂, 67-75%by weight; Na₂O, 10-20% by weight; CaO, 5-15% by weight; MgO, 0-7% byweight; Al₂O₃, 0-5% by weight; K₂O, 0-5% by weight; Li₂O, 0-1.5% byweight; and BaO, 0-1%, by weight.

Other minor ingredients, including various conventional refining aids,such as SO₃, carbon, and the like may also be included in the baseglass. In certain embodiments, for example, glass herein may be madefrom batch raw materials silica sand, soda ash, dolomite, limestone,with the use of sulfate salts such as salt cake (Na₂SO₄) and/or Epsomsalt (MgSO₄×7H₂O) and/or gypsum (e.g., about a 1:1 combination of any)as refining agents. In certain example embodiments, soda-lime-silicabased glasses herein include by weight from about 10-15% Na₂O and fromabout 6-12% CaO, by weight.

In addition to the base glass above, in making glass according tocertain example embodiments of the instant invention the glass batchincludes materials (including colorants and/or oxidizers) which causethe resulting glass to be fairly neutral in color (slightly yellow incertain example embodiments, indicated by a positive b* value) and/orhave a high visible light transmission. These materials may either bepresent in the raw materials (e.g., small amounts of iron), or may beadded to the base glass materials in the batch (e.g., cerium, erbiumand/or the like). In certain example embodiments of this invention, theresulting glass has visible transmission of at least 75%, morepreferably at least 80%, even more preferably of at least 85%, and mostpreferably of at least about 90% (Lt D65). In certain examplenon-limiting instances, such high transmissions may be achieved at areference glass thickness of about 3 to 4 mm In certain embodiments ofthis invention, in addition to the base glass, the glass and/or glassbatch comprises or consists essentially of materials as set forth inTable 1 below (in terms of weight percentage of the total glasscomposition):

TABLE 1 Example Additional Materials In Glass General More MostIngredient (Wt. %) Preferred Preferred total iron 0.001-0.06%0.005-0.04%  0.01-0.03% (expressed as Fe₂O₃): cerium oxide:    0-0.30%0.01-0.12% 0.01-0.07% TiO₂    0-1.0% 0.005-0.1% 0.01-0.04% Erbium oxide:0.05 to 0.5% 0.1 to 0.5% 0.1 to 0.35%

In certain example embodiments, the total iron content of the glass ismore preferably from 0.01 to 0.06%, more preferably from 0.01 to 0.04%,and most preferably from 0.01 to 0.03%. In certain example embodimentsof this invention, the colorant portion is substantially free of othercolorants (other than potentially trace amounts). However, it should beappreciated that amounts of other materials (e.g., refining aids,melting aids, colorants and/or impurities) may be present in the glassin certain other embodiments of this invention without taking away fromthe purpose(s) and/or goal(s) of the instant invention. For instance, incertain example embodiments of this invention, the glass composition issubstantially free of, or free of, one, two, three, four or all of:erbium oxide, nickel oxide, cobalt oxide, neodymium oxide, chromiumoxide, and selenium. The phrase “substantially free” means no more than2 ppm and possibly as low as 0 ppm of the element or material. It isnoted that while the presence of cerium oxide is preferred in manyembodiments of this invention, it is not required in all embodiments andindeed is intentionally omitted in many instances. However, in certainexample embodiments of this invention, small amounts of erbium oxide maybe added to the glass in the colorant portion (e.g., from about 0.1 to0.5% erbium oxide).

The total amount of iron present in the glass batch and in the resultingglass, i.e., in the colorant portion thereof, is expressed herein interms of Fe₂O₃ in accordance with standard practice. This, however, doesnot imply that all iron is actually in the form of Fe₂O₃ (see discussionabove in this regard). Likewise, the amount of iron in the ferrous state(Fe⁺²) is reported herein as FeO, even though all ferrous state iron inthe glass batch or glass may not be in the form of FeO. As mentionedabove, iron in the ferrous state (Fe²⁺; FeO) is a blue-green colorant,while iron in the ferric state (Fe³⁺) is a yellow-green colorant; andthe blue-green colorant of ferrous iron is of particular concern, sinceas a strong colorant it introduces significant color into the glasswhich can sometimes be undesirable when seeking to achieve a neutral orclear color.

It is noted that the light-incident surface of the glass substrate 1 maybe flat or patterned in different example embodiments of this invention.

FIG. 2 is a cross-sectional view of a photovoltaic device (e.g., solarcell), for converting light to electricity, according to an exampleembodiment of this invention. The solar cell of FIG. 2 uses the ARcoating 3 and glass substrate 1 shown in FIG. 1 in certain exampleembodiments of this invention. In this example embodiment, the incomingor incident light from the sun or the like is first incident on optionallayer 3 b of the AR coating 3, passes therethrough and then throughlayer 3 a and through glass substrate 1 and front transparent conductiveelectrode 4 before reaching the photovoltaic semiconductor (active film)5 of the solar cell. Note that the solar cell may also include, but doesnot require, a reflection enhancement oxide and/or EVA film 6, and/or aback metallic or otherwise conductive contact and/or reflector 7 asshown in example FIG. 2. Other types of photovoltaic devices may ofcourse be used, and the FIG. 2 device is merely provided for purposes ofexample and understanding. As explained above, the AR coating 3 mayreduce reflections of the incident light and permits more light to reachthe thin film semiconductor film 5 of the photovoltaic device therebypermitting the device to act more efficiently.

While certain of the AR coatings 3 discussed above are used in thecontext of the photovoltaic devices/modules, this invention is not solimited. AR coatings 3 (which may include just 3 a in certain instances)according to this invention may be used in other applications such asfor picture frames, fireplace doors, and the like. Also, other layer(s)may be provided on the glass substrate under the AR coating so that theAR coating is considered on the glass substrate even if other layers areprovided therebetween. Also, while the first layer 3 a is directly onand contacting the glass substrate 1 in the FIG. 1 embodiment, it ispossible to provide other layer(s) between the glass substrate and thefirst layer in alternative embodiments of this invention.

Set forth below is a description of how AR coating 3 may be madeaccording to certain example non-limiting embodiments of this invention.

Exemplary embodiments of this invention provide a method of making aporous silica coating containing metal oxide(s) for use as the ARcoating 3 (or 3 a), with appropriate light transmission and abrasion orscratch resistance properties. In certain example embodiments of thisinvention, the coating solution may be based on a mixture of at leasttwo sols. The first sol, which is a silica sol, may be based on twodifferent silica precursors, namely (a) a colloidal silica solutionincluding or consisting essentially of particulate silica in a solvent,and (b) a polymeric solution including or consisting essentially ofsilica chains. The second sol, which is or includes a metal oxide sol,may be a polymeric solution containing silica chains as well as a metaloxide, a silane.

In making the polymeric silica solution for the silica sol, a silane maybe mixed with a catalyst, solvent and water. After agitating, thecolloidal silica solution (a) is added to the polymeric silica solution(b), optionally with a solvent. In making the metal oxide sol, a silaneis mixed with a metal oxide, a catalyst, and optionally a complex agent,optionally with a solvent. After agitating the metal oxide sol, it ismixed, combined, and/or agitated with the silica sol. The weightpercentage of the metal oxide sol in the combined sol gel may rangespreferably from greater than 0 to 15% wt, and all subrangestherebetween, more preferably 0.1 to 10% wt, and all subrangestherebetween, more preferably 1 to 5% wt, and all subrangestherebetween, and most preferably about 2% wt. In other alternativeembodiments, greater than 15% wt of the metal oxide sol may be used.

The combined sol gel coating solution is then deposited on a suitablesubstrate such as a highly transmissive clear glass substrate, directlyor indirectly. Then, the sol gel coating solution on the glass 1substrate is cured and/or fired, preferably from about 100 to 750° C.,and all subranges therebetween, thereby forming the solid AR coating 3on the glass substrate 1. The final thickness of the AR coating 3 may,though not necessarily, be approximately a quarter wave thickness incertain example embodiments of this invention. It has been found that anAR coating made in such a manner may have adequate durability, therebyovercoming one or more of the aforesaid mechanical/abrasion resistanceproblems in approaches of the prior art.

In an exemplary embodiment, the sol-gel process used in forming coating3 may comprise: forming a polymeric component of silica by mixingglycycloxypropyltrimethoxysilane (which is sometimes referred to as“glymo”) with a first solvent, a catalyst, and water; forming a silicasol gel by mixing the polymeric component with a colloidal silica, asecond solvent; forming a metal oxide sol by mixing glymo with a metaloxide, a catalyst, water, optionally a complex agent, and a solvent;mixing the silica sol with the metal oxide sol; casting the mixture byspin coating to form a coating on the glass substrate; and curing andheat treating the coating. Suitable solvents may include, for example,n-propanol, isopropanol, other well-known alcohols (e.g., ethanol), andother well-known organic solvents (e.g., toluene). Suitable catalystsmay include, for example, well-known acids, such as hydrochloric acid,sulfuric acid, etc. The colloidal silica may comprise, for example,silica and methyl ethyl ketone. Suitable complex agents may, for exampleinclude acetylacetate, N-methylpyrrolidone, N,N-dimethyl formamide, N,Ndimethyl acrylamide, triethanolamine (TEA), diethanolamine,polyvinylpyrolidone (PVP), citric acid, etc. The mixing of the silicasol and the metal oxide sol may occur at or near room temperature for 15to 45 minutes (and preferably around 30 minutes) or any other periodsufficient to mix the two sols either homogeneously or nonhomogeneously.The curing may occur at a temperature between 100 and 150° C. for up to2 minutes, and the heat treating may occur at a temperature between 600and 750° C. for up to 5 minutes. Shorter and longer times with higherand lower temperatures are contemplated within exemplary embodiments ofthe present invention.

In alternative embodiments, two or more metal sols are mixed with thesilica sol to form a combined sol gel. In further embodiments,additional ingredients, such as organic compounds, may be mixed induring the formation of any of the metal oxide or silica sols, such asdescribed in a co-pending U.S. patent application currently assignedAtty. Dkt. No. 3691-1169, filed Feb. 2, 2007.

The following examples illustrate exemplary, nonlimiting, embodiments ofthe present invention.

Preparation of Silica Sol:

The silica sol was prepared as follows. A polymeric component of silicawas prepared by using 64% wt n-propanol, 24% wtGlycycloxylpropyltrimethoxysilane (Glymo), 7% wt water, and 5% wthydrochloric acid. These ingredients were mixed for 24 hrs. The coatingsolution was prepared by using 21% wt polymeric solution, 7% wtcolloidal silica in methyl ethyl ketone supplied by Nissan ChemicalsInc, and 72% wt n-propanol. This was stirred for 2 hrs to give silicasol. The final solution is referred to as silica sol. Whileglycycloxypropyltrimethoxysilane is an example silane that is used,other silane(s) may instead be used in different embodiments of thisinvention.

Preparation of Mullite (3Al₂O₃:2SiO₂) Sol:

Mullite sol containing 3 parts alumina and 2 parts silica was preparedby taking 2.18 gm aluminum tert butoxide and 0.73 gmGlycycloxylpropyltrimethoxysilane (Glymo) in a solution containing 6 gmacetylacetate, 6 gm hydrochloric acid, and 20 gm n-propanol. Thissolution was stirred for 15 minutes. Then 0.5 gm water was added. Thesolution was stirred for another 15 minutes. The final solution isreferred to as 3Al₂O₃:2SiO₂ sol.

Preparation of Sillimanite (Al₂O₃:SiO₂) Sol:

Sillimanite sol containing 1 part alumina and 1 part silica was preparedby taking 2.45 gm aluminum tert butoxide and 1.15 gmGlycycloxylpropyltrimethoxysilane (Glymo) in a solution containing 2 gmacetylacetate, 6 gm hydrochloric acid, and 20 gm n-propanol. Thissolution was stirred for 15 minutes. Then 0.5 gm water was added. Thesolution was stirred for another 15 minutes. The final solution isreferred to as Al₂O₃: SiO₂ sol.

Preparation of Spinel (Mg Al₂O₄) Sol:

Spinel sol containing 1 part magnesia (MgO) and 1 part alumina wasprepared by taking 2.5 gm aluminum tert butoxide and 1.07 gm magnesiumacetate in a solution containing 2 gm acetylacetate, 6 gm hydrochloricacid, and 20 gm n-propanol. This solution was stirred for 15 minutes.Then 0.5 gm water was added. The solution was stirred for another 15minutes. The final solution is referred to as Mg Al₂O₄ sol.

Preparation of Alumina (Al₂O₃) Sol:

2.52 gm aluminum tert butoxide was mixed in a solution containing 2 gmacetylacetate, 6 gm hydrochloric acid, and 20 gm n-propanol. Thissolution was stirred for 15 minutes. Then 0.5 gm water was added. Thesolution was stirred for another 15 minutes. The final solution isreferred to as Al₂O₃ sol.

Preparation of Zirconia (ZrO₂) Sol:

3.8 gm of zirconium butoxide was mixed in a solution containing 2 gmacetylacetate, 6 gm hydrochloric acid, 2 gm nitric acid, and 20 gmn-propanol. This solution was stirred for 15 minutes. Then 0.5 gm waterwas added. The solution was stirred for another 15 minutes. The finalsolution is referred to as ZrO₂ sol.

Preparation of Anorthrite (CaAl₂Si₂O₈) Sol:

Anorthrite sol containing 1 part alumina, 2 parts silica and 1 partcalcium oxide, was prepared by taking 2.5 gm aluminum tert butoxide, 2.3gm Glycycloxylpropyltrimethoxysilane (Glymo) and 0.8 gm calcium acetatein a solution containing 2 gm acetylacetate, 6 gm hydrochloric acid, and20 gm n-propanol. This solution was stirred for 15 minutes. Then 0.5 gmwater was added. The solution was stirred for another 15 minutes. Thefinal solution is referred to as CaAl₂Si₂O₈ sol.

Measurement of Scratch Resistance:

Scratch resistance of the coatings was tested using the Micro-Tribometermanufactured by CETR (Center for Tribology) using pre-cleanedborosilicate glass (BSG) media with nominally 3.2 mm diameter. Theapplied coating under test was also cleaned using isopropanol and wipedgently by soft Shurwipes prior to testing. Constant load mode wasselected to evaluate the AR coatings.

The loading process is described as follows. The BSG media is insertedinto a media holder which is attached to a load sensor (in our case, weused 50 gm or nearly 500 mN load sensor). The carriage holding the loadsensor was allowed (as per our pre-programmed instruction) to traveldownward to allow landing the media onto the specimen surface at apre-determined load (in our case, it was between 20-130 mN), followed bya pre-determined speed of travel (in our case it was 30 mm in 60 secs)of the specimen stage. Each scratch process was followed by visual andmicroscopic examination of the coating surface to look for scratchdamage. The process was repeated a few times for each coating. The loadat which the scratch becomes visible is defined a critical scratch load.

EXAMPLE #1

The silica coating was fabricated using spin coating method with 1000rpm for 18 secs. The coating was heat treated in furnace at 625° C. forthree and a half minutes. The critical scratch load of this coating is30 mN as shown in the table 2.

EXAMPLE #2

The example #2 is same as example #1 except 3Al₂O₃:2SiO₂ sol and silicasol were taken in 2:98 percent weight ratio respectively and mixed for30 mins at room temperature prior to spin coating. The critical scratchload of this coating is 50 mN as shown in the table 2.

EXAMPLE #3

The example #3 is same as example #2 except the Al₂O₃:SiO₂ sol andsilica sol were taken in 2:98 percent weight ratio respectively. Thecritical scratch load of this coating is 70 mN as shown in the table 2.

EXAMPLE #4

The example #4 is same as example #2 except the Mg Al₂O₄ sol and silicasol were taken in 2:98 percent weight ratio respectively. The criticalscratch load of this coating is 40 mN as shown in the table 2.

EXAMPLE #5

The example #5 is same as example #2 except the ZrO₂ sol and silica solwere taken in 2:98 percent weight ratio respectively. The criticalscratch load of this coating is 80 mN as shown in the table 2.

EXAMPLE #6

The example #6 is same as example #2 except the Al₂O₃ sol and silica solwere taken in 2:98 percent weight ratio respectively. The criticalscratch load of this coating is 70 mN as shown in the table 2.

EXAMPLE #7

The example #7 is same as example #2 except the CaAl₂Si₂O₈ sol andsilica sol were taken in 2:98 percent weight ratio respectively. Thecritical scratch load of this coating is 100 mN as shown in the table 2.

TABLE 2 Coating of derived from porous silica and additives Metal oxidesol Silica sol Scratch invisible by Example No. (% wt) (% wt) eyes (CRL)Example #1 None 100 30 mN Example #2 3Al₂O₃:2SiO₂ 98 50 mN 2 Example #3Al₂O₃:SiO₂ 98 70 mN 2 Example #4 Mg Al₂O₄ 98 40 mN 2 Example #5 ZrO₂ 9880 mN 2 Example #6 Al₂O₃ 98 70 mN 2 Example #7 CaAl₂Si₂O₈ 98 100 mN  2

Table 2 illustrates that approximately 40-70 mN critical scratch loadcan be obtained using mullite, silimanite and spinels; approximately70-80 mN (or 60-90 nM) critical scratch load can be obtained usingalumina and zirconia; and approximately 100 mN (e.g., from about 80-120mN) critical scratch load can be obtained using tri-metal oxides such asanorthrite.

All numerical ranges and amounts are approximate and include at leastsome variation.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

The invention claimed is:
 1. A photovoltaic device comprising: aphotovoltaic film, and at least a glass substrate on a light incidentside of the photovoltaic film; an anti-reflection coating provided onthe glass substrate; wherein the anti-reflection coating comprises atleast a layer provided directly on and contacting the glass substrate,wherein the layer comprises porous silica and a metal oxide and whereinthe layer comprising porous silica and the metal oxide comprises lessthan 5% of the metal oxide by weight, and wherein the layer comprisingporous silica and the metal oxide comprises anorthite.
 2. Thephotovoltaic device of claim 1, wherein the glass substrate comprises asoda-lime-silica glass including the following ingredients: SiO₂, 67-75%by weight; Na₂O, 10-20% by weight; CaO, 5-15% by weight; MgO, 0-7% byweight; Al₂O₃, 0-5% by weight; K₂O, 0-5% by weight; Li₂O, 0-1 5% byweight; and BaO, 0-1%, by weight.
 3. The photovoltaic device of claim 1,wherein the metal oxide further comprises one or more of: 3Al₂O₃:2SiO₂,Al₂O₃: SiO₂, Mg Al₂O₄, ZrO₂, and Al₂O₃.
 4. The photovoltaic device ofclaim 1, wherein the anti-reflection coating further comprises a layercomprising an oxide of silicon located over and contacting the layercomprising porous silica and the metal oxide.
 5. The photovoltaic deviceof claim 1, wherein the layer comprising porous silica and the metaloxide comprises a critical scratch load of from about 40 to 70 mN. 6.The photovoltaic device of claim 1, wherein the anti-reflection coatingcomprises a critical scratch load of from about 40to 70mN.
 7. Thephotovoltaic device of claim 1, wherein the layer comprising poroussilica and the metal oxide comprises a critical scratch load of fromabout 70 to 80 mN.
 8. The photovoltaic device of claim 1, wherein theanti-reflection coating comprises a critical scratch load of from about70 to 80 mN.
 9. The photovoltaic device of claim 1, wherein the layercomprising porous silica and the metal oxide comprises a criticalscratch load of from about 80 to 120 mN.
 10. The photovoltaic device ofclaim 1, wherein the anti-reflection coating comprises a criticalscratch load of from about 80 to 120 mN.
 11. A photovoltaic devicecomprising: a photovoltaic film, and at least a glass substrate on alight incident side of the photovoltaic film; an anti-reflection coatingprovided on the glass substrate; wherein the anti-reflection coatingcomprises a first layer comprising porous silica and a metal oxideprovided directly on and contacting the glass substrate, wherein thefirst layer comprises anorthite; and a second layer comprising an oxideof silica provided over and contacting the first layer, wherein theanti-reflection coating comprises a critical scratch load of from about40 to 120 mN.
 12. The photovoltaic device of claim 11, wherein the firstlayer further comprises mullite, silimanite, and/or spinels.
 13. Thephotovoltaic device of claim 11, wherein the first layer comprises lessthan about 5% metal oxide by weight.
 14. The photovoltaic device ofclaim 1, wherein the layer comprising porous silica and the metal oxidecomprises about 2% metal oxide by weight.
 15. The photovoltaic device ofclaim 13, wherein the first layer comprises about 2% metal oxide byweight.